Which year saw the greatest contribution to physics by any individual?
But GR is the greatest.
I'd have to go with 1666. Comparing the intellectual climate of both eras, Newton had a much tougher trail to blaze, due to his lack of fellow researchers, as well as the fact that he didn't even have the mathematical tools in place to formulate his theories. Not that einstein picked SR off the shelf or anything, but he did have a significant supporting cast.
But the question is about Newton or SR...
1686. vs 1915. ?
To throw in another question ^
Which of those years more underlies today's physics overall?
Theoretical physics really had it's roots with Newton, so i'd say 1666.
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